For a more apples-to-apples comparison, have you tried changing the Speaker Impedance Curve to "Resistive Load" or one of the "Load Box" curves? Theoretically, that should put it on closer to a level playing field with your amps using a reactive load.
That said, the true ideal is a tube amp into a real speaker. In that sense, the Fractal modeling has an advantage that it can model power amp interactions that aren't happening with a load box.